1. Field of the Invention
This invention relates to the cleaning of aluminum surfaces, particularly drawn and ironed aluminum cans containing lubricant contaminants, using an alkaline composition.
2. Statement of the Related Art
Containers of aluminum and aluminum alloys are manufactured by a drawing and forming operation, commonly referred to as drawing and ironing. This operation results in the deposition of lubricant and forming oil contaminants on the surfaces of the container. In addition, residual aluminum fine contaminants are deposited on the surfaces, with relatively larger quantities present on the inside surface of the container.
Prior to processing the containers, e.g. conversion coating and sanitary lacquer deposition, the surfaces of the containers must be clean and free of waterbreaks, so that no contaminants remain on the surfaces which will interfere with further processing of the containers.
Compositions currently used commercially for cleaning such aluminum containers are aqueous sulfuric acid solutions containing hydrofluoric acid and one or more surfactants. Such cleaning solutions are quite effective and have many advantages. However, there are also some disadvantages associated with such acid cleaning compositions. For example, such compositions are capable of dissolving stainless steel and other iron alloy equipment commonly utilized in the container cleaning lines. Also, hydrofluoric acid and fluorides present in spent cleaning baths and rinse water present an environmental problem in their disposition.
Alkaline cleaning solutions have been formulated in the past to try to overcome the above problems, but such alkaline solutions have instead raised new serious problems of their own which have mitigated against their commercial use. For example, when cleaning solutions employing alkali metal hydroxides were tried, extensive and irregular etching of the aluminum containers occurred, rendering the containers commercially unacceptable.
Other alkaline cleaning solutions have also been tried with varying success. For example, U.S. patent application Ser. No. 273,484 and a continuation-in-part thereof, Ser. No. 383,289 disclosed an alkaline cleaner comprising: 0.5 to 3 grams/liter (g/l) of an alkali metal hydroxide (such as NaOH); 1 to 5 g/l of an alkali metal salt of ethylenediaminetetraacetic acid (such as sodium EDTA); 0.1 to 10 g/l of at least one anionic, cationic, or nonionic surfactant (such as an anionic surfactant believed to be composed of two parts of a modified polyethoxylated straight chain alcohol and one part of a linear alkyl succinate, optionally combined with an alkali metal salt of 2-butoxyethoxyacetate); and optionally further containing 0.6 to 1.3 g/l of an aluminum sequestering agent (such as sodium glucoheptonate). It may be noted that the EDTA in this composition does not function as an aluminum sequestering agent, because of the alkaline pH of the composition.
While the compositions of the above applications were excellent aluminum can cleaners, resulting in cans with virtually no waterbreaks, problems arose when a production line was interrupted for any length of time beyond a few minutes. It was found that cans that stood without after rinsing for any length of time developed severe staining, particularly at those points where the cans were in contact with each other. Even the slightest such stain would make the cans unusable, since they appeared blemished, even after subsequent coating. While most can cleaning operations are by spraying with a cleaner for a short time such as 10 to 60 seconds, it was also found that times of 60 to 120 seconds, which are occasionally employed, might also result in staining. Additionally, it was found that where there was an usually large amount of lubricant contaminant, such as more than about 1.5 g/l, the cleaner was less effective.
A number of patents or published patent applications disclose alkaline or neutral cleaning compositions for metal surfaces, including the following:
U.S. Pat. No. 3,975,215--Rodzewich, assigned to Amchem Products, Inc.
U.S. Pat. No. 3,888,783--Rodzewich, assigned to Amchem, Products, Inc.
U.S. Pat. No. 4,093,566, assigned to the United States of America
Japanese No. 53/149,130, assigned to Nihon Parkerizing
Japanese No. 51/149,830, assigned to Matsushita Elc. Ind.
Japanese No. 50/067,726, assigned to Kurita Water Ind.
Japanese No. 48/103,033, assigned to Nittan Co., Ltd.
Prior art acid cleaning composition for cleaning aluminum surfaces are disclosed in U.S. Pat. No. 4,124,407--Binns, U.S. Pat. No. 4,116,853--Binns, U.S. Pat. No. 4,009,115--Binns, and U.S. Pat. No. 3,969,135--King.
U.S. Pat. No. 4,477,290 assigned to Pennwalt, describes an alkaline aluminum cleaner having a minimum amount of 6 g/l of NaOH or KOH, which is far in excess of a desirable amount and will cause smutting. The solutions are stated as having a pH of about 13. Chelating (sequestrant) agents including sorbitol, gluconic acid, and glucoheptoic acid are disclosed. A composition of 0.6 to 2 g/l of tetrapotassium pyrophosphate, 0.5 to 1.8 g/l of sodium gluconate, and 0.5 to 1.8 g/l of KOH is also disclosed, although no EDTA or surfactant is present.